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Aspects of Zero-Valent Nickel, Palladium and Platinum Chemistry

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Aspects of Zero-Valent Nickel, Palladium and Platinum Chemistry ASPECTS OF ZERO-VALENT NICKEL, PALLADIUM AND PLATINUM CHEMISTRY DEDICATION To my wife, Ethel, my daughter, Cheryl and my parents, Mr. and Mrs. Robert Mac Donald for their continual support and encoumg.ement. ASPECTS OF ZERO-VALENT NICKEL, PALLADIUM AND PLATINUM CHEMISTRY By ROBERT RICHARD MACDONALD B.Sc. A Thesis Submitted to the Faculty of Graduate Studies in Portia I Fulfilment of the Requirements for the Degree Master of Science McMaster University November 1974 MASTER OF SCIENCE (1974) McMASTER UNIVERSITY (Chemistry) HAMILTON, Ontario TITLE: Aspects of Zero-Valent Nickel, Palladium and Platinum Chemistry AUTHOR: Robert Richard MacDonald, B.Sc. (Laurentian University) SUPERVISOR: Or. P. M. Mai ti is NUMBER OF PAGES: v, 97 SCOPE AND CONTENTS: The work describes the preparation and properties of some novel dibenzylideneacetone complexes of palladium and platinum. The structure and bonding in these complexes is discussed in view of their infrared and nuclear magnetic resonance spectra. The cyclotrimerization of acetylenes by zero-valent nickel complexes is discussed and the synthesis of a new cyclobutadiene­ platinum complex is reported. (ii) ACKNOWLEDGEMENTS The author is endebted to Dr. P. M. Mai ti is for his continuing advice and interest shown in the work. Thanks are also extended to Dr. M. J. Mc GI inc hey for help and advice in the preparation of this thesis. (iii) CONTENTS Page INTRODU:TION 1. Bonding in Zero-Valent Complexes .••••••••••••••••••• l 1. l Bis and Tris(dibenzylideneacetone) Complexes of Palladium and Platinum...................... 3 1.2 PropertieSaooeooooooo•••o•o•ooo••••o•o•ooeoooo 3 1.3 ReactionSoeoooooeoo•ooo•ooooooeo•o•••ooooooo• 9 1.4 Reaction Mechanism: Oligomerization of Acetylenes by Pd (O) Complexes................ 17 2. Cyclobutadiene-Metal Complexes of Nickel, Palladium and Platinumo o.. o. o.•...•••••..•..•• o.... 22 2. l Pre para tion .• o • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 23 2.2 Properties of Cyc lobutadi ene-MetaI Comp Iexes. • • • 29 2.3 Reactions of Cyclobutadiene-Metal Complexes.... 31 2.4 Cyclobutadiene Complexes as Reaction Intermediates in Cyclotrimerization Reactions..... 34 RESULTS AND DISCUSSION.................................. 37 C 0 NC LUSI 0 NS ••••••••• ., •••••••••••••• o • Q • • • • • • • • • o •••• o • • 62 EXPERIMENTAL ••••••••• o. o.............. o.. o............... 63 App EN DI x0 •• 0 0 0 • 0 0 0 0 0 0 • 0 0 0 D 0 0 0 •• 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 • 0 0 ••• 0 • 0 84 BI BL I OG 'RAP r1v. 0 I) 0 0 • 0 0 0 0 0 0 0 0 0 • 0 0 0 • 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 • • ~ 93 (iv) Table of Abbreviations Abbreviation Compound dba di benzyl i deneac etone dma dimethyl acetylene dicarboxylate o-phen o-phenanthroline tmedta tetramethylethyl enediam i ne PY pyridine dmg dimethylglyoxime dte 1,2-bis(methylthioethane) badn biacetyl-dianii para-methoxy I dba bis(para-methoxy)dibenzylideneacetone para-fIuoro dba bis(para-fluoro )dibenzyl ideneacet.one para-dimethylamino dba bis(para-dimethylamino )dibenzyl i­ deneacetone para-methyl dba bis(para-methyl )dibenzyl ideneacetone dbh 2,6-dibenzylidenecyc lohexanone v INTRODUCTION INTRODUCTION In. an effort to rationalize the mechanisms of heterogeneous catalysis, considerable attention has been focussed recently on the chemistry of low valent metal complexes. This presupposes that coordination and chemisorption are interrelated and that the behaviour of low valent metal complexes in solution is essentially that of solvated metal atoms. Zero-valent d10 systems are particularly interesting due to their tendency to stabilize coordinatively unsaturated compounds. One series of compounds in particular, the dibenzylideneacetone (dba) complexes of palladium and platinum, has generated widespread interest and the subsequent work helped to elucidate some of the steps involved in the cyclotrimerization of acetylenes. The investigation described in this thesis is devoted to the study of the preparation, properties and reactions of the dba complexes and their products. 1. Bonding in Zero-valent Complexes The d-block transition metals are characterized by their ability to form complexes with a wide variety of neutral molecules such as carbon monoxide, isocyanides, substituted phosphines, arsines and stibines, pyridine, 2,2'bipyridyl, etc •• The types of complexes range from simple binary compounds such as Ni (C0)4 to mixed species such as (Ph3P) 2 Pt(CO)~ and 2 (Ph3P)2Pd(SbPh 3)2 • In many of these complexes, the metal is formally in a zero • 2 . oxidation state. The metal already has a full complement of electrons and, on bonding to a neutral ligand, must have some mechanism for dissipating the build-up of negative charge which takes place. This charge build-up on the metal from the fonnation of the coordinate bond is believed to be removed by the back donation of electron density from filled metal d orbitals to vacant ligand w* or d orbitals by the formation of . Tr-type bonds. 3 ~1~ -~r~X-• • a b The bonding in complexes involving trivalent phosphorus arsenic, antimony and bismlith and divalent sulphur and selenium ligands can be explained by back donation to empty d orbitals of correct symmetry as illustrated in figure {a). Back donation from the metal to ligands such as carbon monoxide, bipyridyl, isonitriles, etc. involves the use of vacant ligand 1T * orbitals for dissipation of electron density as ii lustrated in figure (b). 3 1. 1 Bis and His (dibenzylideneacetone)-Complexes of Palladium and Platinum The first of these complexes, bis(dibenzylideneacetone) ­ palladium, (dba)2Pd, was reported in 1970 by Takahashi et aL4 Previously, the only known zero-valent complexes of palladium involved ligands such as phosphines5-7, phosphites5, arsines5, isocyanides6 and carbon monoxide8-10. Subsequently, Moseley and Mai ti is 11 reported the synthesis of the platinum complexes (dba)2Pt and (dba)JPt. Mazza and Pierpont12 reported the synthesis and crystal structure of the remaining member of this series, (dba) Pd. 3 Independently Ishii et al. l3 and Mazza and Pierpont14 reported the synthesis and crystal structure of an interesting new complex Pd2(dba)3.solvent • This was the only product recovered from crystallization of a solution of (dba)2Pd in solvents such as chloroform, methylene chloride, benzene, toluene and THF. l .2 Properties There are two divergent opinions as .to the actual structure of (dba) P<l41 11. Takahashi suggested that the dba was co-ordinated 2 to the Pd0 mainly through the c=c bonds. This was supporte.d by i.r. evidence; the absorption bands were similar to those of dba, but the bands at 1627 and 983 cm:.. l (C=c) had disappeared and ...J (C=O) 4 had shifted from 1651 {free dba) to 1620 cm-1 {co-ordinated dbc). In a subsequent spectral comparison of dba, {dba)2Pd and (dba)2Pt, Moseley and Maitlis11 proposed that in solution, the metal atoms were bonded to the carbonyl groups rather than the double bonds as originally proposed. They found that in the i .r. spectra in chloroform, .J (C=O) of both complexes had almost disappeared while ..J (C=c) remained intense and relatively unchanged. Two new bands at 1544 (Pd) and 1527 (Pt) cm-1 were observed and identified as modified ..) (C=O) bands. The electronic spectra of the complexes were similar to dba but had intense charge-transfer bands attributed to metal-ligand (d-1T*) transitions. In addition, the n.m.r. spectrum of {dba)2Pt was identical to that of dba itself with no evidence of any coupling to 195pt (I=~, 34% abundance) by either olefinic or aromatic protons. Although (dba)2Pd was only slightly soluble in the solvents used, similar results were obtained. The proposed structure is pictured below. 5 Most of the spectra quoted in the reports were taken in methylene chloride of chloroform. For this reason, the crystallization product -~d2 (dba)3.solvent l3, 14 provides interesting infonnation concerning the nature of the Pd - dba complex in solution. Mazza and Pierpont14 recorded the i.r. spectra of both the initial (dba)2Pd solution and a solution of Pd2(dba)3.solvent in chloroform and methylene chloride. The spectrum of the initial complex resembled that of Moseley and Mai ti is 11; however, the spectrum of Pd2(dba)3. solvent resembled that of Takahashi4 with .J (C=O) at 1618 cm-1 and olefinic bands at 1580, 1548 and 1541 cm-1. Ishii et ai. 13 conducted a more detailed study of the complex. Binuclear complexes with the ligands decadeuteriodibenzyl ideneacetone, (C6DsCH=CH)2CO, and p,p-dimethoxydibenzylideneacetone were a~so prepared and studied. Differences in the i. r. spectra of Pd2(dba)J.solvent and Pd2(dba-d10)3.solvent as compared to dba and dba-d10 were as follows: a characteristic VCH(trans) vibration observed at 983 cm-1 in the free ligands almost completely disappeared in the complexes, although a broad weak band appeared at a slightly lower frequency (975 cm-1) in both. Also, the ..J (C=O) bands 6 were shifted about 40 cm-1 to lower frequency. The .J (C-Cl) bands, hidden in the spectrum of Pd2(dba)3. solvent by the strong .J CH(arom) absorptions, appeared at 766 and l47 cm-1 in the deuterated complex. The electronic spectrum of Pd2(dba) .solvent showed bands 3 assigned to n - TT* and TT - TT* transitions at 234 and 321 m..M. • These remained unchanged from the transitions observed for the free ligand. 1 The H n.m.r. spectrum showed a multiplet centred at 3.51"', identified as the ten-ring proton resonance of the co-ordinated dba. This was confirmed by the disappearance of this multiplet in the n.m.r. spectrum of the complex containing the deuterated ligand. Olefinic proton resonances of the co-ordinated dba were spread over the range from 2.5 to 5.5 'I .
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